Which of the following services enables logging on to one computer system and working on another?

The correct answer is Telnet because it is a network protocol that allows a user to log into and interact with a remote computer system over a TCP/IP network. When using Telnet, you can access command-line functions of the remote computer, as though you were using it directly, despite being physically located on another machine. This ability to log into one system and operate on another is fundamental for remote management, troubleshooting, and accessing services hosted on different servers.

In contrast, File Transfer Protocol (FTP) primarily facilitates the transfer of files between machines but does not provide command-line access to the remote system. LISTSERV is an application for managing email lists, useful for disseminating information but not for remote computing. The World Wide Web refers to information accessed via web browsers, focusing on retrieving and viewing content rather than providing direct access to a remote system's operating environment.